A "Gem" of a Meteor Shower is Coming

The Leonid Meteor Shower has long passed us by, but that doesn’t mean that there aren’t any good meteor displays to look forward to. In fact, one of the best is just around the corner, scheduled to reach its peak during the early hours of Tuesday, Dec. 14: The Geminid Meteors.

The Geminids get their name from the constellation of Gemini, the Twins. During the overnight hours of December 13-14—the night of this shower’s maximum—the meteors appear to emanate from a spot in the sky near the bright star Castor in Gemini.

Best of the Best

The Geminid Meteors are—for those willing to brave the chill of a December night—a fine winter shower, and usually the most satisfying of all the annual showers, even surpassing the famous Perseid meteors of August. Studies of past displays show that this shower has a reputation for being rich both in slow, bright, graceful meteors and fireballs as well as faint meteors, with relatively fewer objects of medium brightness. Many appear yellowish in hue. Some even seem to form jagged or divided paths.

According to the late meteor specialist Neil Bone (1959-2009), at 2 grams per cubic centimeter on average, Geminid meteoroids are several times denser than the cometary dust flakes that supply most meteor showers. Add this to the relatively slow speed with which Geminids typically encounter Earth—22 miles (35 km) per second—roughly half the speed of a Leonid meteor and you have the recipe for meteors that linger a bit longer in view than most.

A map of the sky showing the radiant of the Geminid meteor shower.

Credit: StarryNight Software

The Earth moves quickly through this meteor stream. Rates increase steadily for two or three days before maximum, reaching roughly above a quarter of its peak strength, then drops off more sharply, lasting for only about a day afterward. Those late Geminids, however, tend to be especially bright. A few renegade forerunners and late stragglers might be seen for a week or more before and after the peak night. One interesting finding made recently from video analysis by the International Meteor Organization (IMO) was that Geminids have been detected as early as Nov. 30—totally unexpected from past visual observations.

Some Lunar Interference

The Geminids perform splendidly in any year, although as was the case for last month’s Leonids, once again the moon is going to pose a bit of a problem. In fact, the moon will reach First Quarter phase on Dec. 13, the very same night as the Geminid peak, shining brightly in the dim constellation of Pisces, the Fishes. That means that many of the fainter Geminid streaks will likely be washed out by the bright moonlight.

But unlike the Leonids, where the Moon was brightly illuminating the sky most of the night, in the case of the Geminids the Moon will be setting at around 12:30 a.m. local time early on Tuesday, Dec. 14. That means that the sky will be dark and moonless for the balance of the night, making for perfect viewing conditions for the shower.

Perfect Timing

In addition, according to Margaret Campbell-Brown and Peter Brown in the 2010 Observer’s Handbook of the Royal Astronomical Society of Canada, peak activity is projected to occur at or near 6 a.m. EST on Dec. 14. Under normal conditions on the night of maximum activity, with ideal dark-sky conditions, at least 60 to 120 Geminid meteors can be expected to burst across the sky every hour on the average (Light pollution greatly cuts the numbers). So, in 2010, along with the lack of significant moonlight, North Americans are projected to be the best situated to catch the very crest of the shower, when the hourly rates conceivably could exceed 120.

But British meteor astronomer, Alastair McBeath also points out that a detailed new analysis confirms that Geminid near-peak activity is very persistent with hourly rates of around 80 to 130 are often seen for almost a day around the predicted time of maximum, corresponding this year from roughly 19h UT on Dec. 13 to 16h UT on Dec. 14. So from virtually anywhere on earth, an excellent Geminid show can be anticipated.

Bundle Up

A productive Geminid watch can actually begin as early as 10 p.m. local time, because the shower’s radiant is already fairly high in the eastern sky by then. Even with that annoyingly bright Moon still high in the western sky, it will be worth watching for some early Gems.

But keep this in mind: at this time of year, meteor watching can be a long, cold business. You wait and you wait for meteors to appear. When they don't appear right away, and if you're cold and uncomfortable, you're not going to be looking for meteors for very long! Therefore, make sure you're warm and comfortable. Warm cocoa or coffee can take the edge off the chill, as well as provide a slight stimulus. It's even better if you can observe with friends. That way, you can keep each other awake, as well as cover more sky. Give your eyes time to dark-adapt before starting.

Debris from a Dead Comet?

The Geminids will be especially noticeable right after the Moon sets, as their radiant point will be passing very nearly overhead. The higher a shower’s radiant, the more meteors it produces all over the sky. The track of each one does not necessarily begin near Castor, or even in the constellation Gemini, but it always turns out that the path of a Geminid extended backward along the direction of flight passes through a tiny region of sky about 0.2-degree in diameter (an effect of perspective). In apparent size, that’s less than half the width of the Moon. As such, this is a rather sharply defined radiant, as meteor showers go, suggesting the stream isyoung—perhaps only several thousand years old.

Geminids stand apart from the other meteor showers in that they seem to have been spawned not by a comet, but by 3200 Phaeton, an Earth-crossing asteroid. Then again, the Geminids may be comet debris after all, for some astronomers consider Phaeton to really be the dead nucleus of a burned-out comet that somehow got trapped into an unusually tight orbit.